1. Field of the Invention
The invention relates to a correlation value calculator, and more particularly, to a correlation value calculator suitable for use in a mobile communication system in which a process for detecting a correlation value is required to be performed in a limited time.
2. Description of Related Art
Among the present available communicating techniques, there is a spread spectrum communication system in which transmission data is spread, for the transmission, to a signal having a band that is sufficiently wider than that of the transmission data. As a system in which this kind of communication system is used for the mobile communication, there is TIA/EIA/IS-95A CDMA mobile communication system that has been standardized in North America.
FIG. 1 shows a basic structure of the CDMA mobile communication system. The CDMA mobile communication system of FIG. 1 exemplifies a system constituted by three fixed radio transmitters (hereinafter, referred to as xe2x80x9cbase stationsxe2x80x9d) 1 to 3 and one mobile station 4. Radio zones 5 to 7 capable of communicating with the base stations 1 to 3 are shown by circles in the diagram. It is assumed that transmission data which is spread spectrum encoded by a base station identification code that is peculiar to each station is transmitted from each of the corresponding base stations 1 to 3 to the radio zones 5 to 7, respectively.
In the case of FIG. 1, the mobile station 4 is in a state in which a speech/data communication with the base station 1 has been established (that is, a state where it is synchronized with the base station identification code of the base station 1) and executes a demodulating process and a hand-off control of a reception signal on the basis of a detecting process of a multi-path signal which arrives from the base station 1 and signals which arrive from the adjacent base stations 2 and 3 and a measuring process of their signal intensities.
The outline of a signal detecting procedure and a signal intensity measuring procedure which are executed in the CDMA communication system will be described hereinafter. The processes (a) through (f) are executed in order, which are as follows.
(a) The base station 1 transmits a phase code range (SRCH_ASET_WIN_SIZE) which is presumed such that the muti-path signal can be detected by the mobile station 4.
(b) Based on the information given by (a), the mobile station 4 generates a spreading code of the base station 1 and obtains a correlation with the reception signal at each code phase.
(c) The mobile station 4 then allocates, to a demodulator (not shown), a spreading code phase of which the correlation value obtained in (b) is equal to or higher than a demodulation enabling level, and performs the demodulation respectively.
(d) The base station 1 transmits spreading code information (pilot PN offset) of the adjacent base stations 2 and 3 and a code phase range (SRCH_NSET_WIN_SIZE) which is presumed such that the signals from the adjacent base stations 2 and 3 can be detected to the mobile station 4.
(e) Based on the information given by (d), the mobile station 4 generates spreading codes of the adjacent base stations 2 and 3 and obtains a correlation with the reception signal at each code phase.
(f) When the correlation value obtained in (e) exceeds a threshold value (T_ADD) which has been predetermined from the base station 1, the mobile station 4 determines that the receivable adjacent base station signals could be demodulated, thereby reporting its fact to the base station 2.
To perform the signal detecting process and the signal intensity measuring process as mentioned above, in the receiver, the correlation between the spreading code which is generated in the receiver and the reception signal needs to be calculated with respect to all of the code phases which are designated from the base station.
A sliding correlation system is generally used currently as a correlation calculating means for this purpose. The system is operative to calculate a correlation with the reception signal while a spreading code phase on the receiver""s side is being shifted.
FIG. 2 shows the principle of calculation of a sliding correlation. Particularly, FIG. 2 shows the principle of calculation in a case where a correlation length is equal to xe2x80x9c4xe2x80x9d and a search phase width, i.e., the number of phases in the range (24 code bits in FIG. 2) in which the correlation is searched is equal to xe2x80x9c6xe2x80x9d . The figure shows operation contents performed when shifting the spreading code phase that is generated by the receiver by one phase each time the correlation calculation of the spreading code phase is performed.
FIG. 3 shows a structure of a reception signal system of the mobile station 4 having a sliding correlator 14 which operates in accordance with the above scheme. As shown in FIG. 3, the mobile station 4 has a reception signal system which is constituted by an antenna 11, an RF demodulator 12, a filter 13, the sliding correlator 14, a CPU 15, demodulators 16-1 to 16-N, and a combiner 17.
In the mobile station 4, the signal received by the antenna 11 is converted from a radio propagating frequency to a baseband signal by the RF demodulator 12 and is further subjected to waveform shaping by the filter 13. By supplying the waveform-shaped baseband signal to the sliding correlator 14 and a plurality of demodulators 16-1 to 16-N, respectively, the following searching process (a self base station searching process and an adjacent base station searching process) is performed.
Firstly, at the time of a self base station searching process, the sliding correlator 14 generates a spreading code and a pilot spreading code of the base station (self base station) currently communicating with, that is designated by the CPU 15, then obtains a correlation with an input signal while shifting a code timing in a phase range and a correlation length which are designated by the CPU 15, and supplies the obtained correlation value to the CPU 15. When the CPU 15 selects a receivable multi-path signal timing on the basis of the obtained correlation value, the CPU supplies code timing information to each of the demodulators 16-1 to 16-N. The demodulators 16-1 to 16-N generate the base station spreading code and data spreading code at the reception timing which is designated by the CPU 15 and demodulate reception data. The demodulated reception data is maximum ratio (M.R.) combined by the combiner 17 and is supplied as final reception data to the CPU 15.
At the time of the adjacent base station searching process, the sliding correlator 14 generates the spreading code and pilot spreading code of the adjacent base station designated by the CPU 15, obtains a correlation with the input signal while shifting the code timing in a phase range and a correlation length which are designated by the CPU 15 and supplies the obtained correlation value to the CPU 15. The CPU 15 discriminates or determines the presence or absence of the hand-off capable base station and its timing on the basis of the obtained correlation value. When the presence of the hand-off capable base station is determined, the CPU 15 notifies the self base station of this discrimination result.
As mentioned above, the sliding correlator 14 needs to always operate to search (multi-path search) the signal which arrives from the self base station through a different transmission path and to search a signal which arrives from the adjacent base station.
Although the sliding correlator 14 with the above construction has a feature such that it can be realized by a simple circuit construction, however, there is a problem that it takes a long time to calculate all of the correlations of the spread phase range which is designated from the base station because of a nature such that the correlation is calculated while shifting the phase. Moreover, as a result of the time needed to calculate the correlations as mentioned above, if a situation occurs such that the detection of the signal which arrives from the adjacent base station is delayed , there is a possibility that the hand-off fails and the shut-off of the communication occurs.
The invention is made in consideration of the above problems and intends to provide a correlation value calculating apparatus which can realize a reduction of a time that is required for a correlation calculating process and can efficiently perform a detection of a signal and a measurement of its intensity.
To solve the above problem, in the invention, there is provided a correlation value calculating apparatus in which by multiplying a reception signal by a spreading code, a correlation value which gives a correlation degree for the spreading code is calculated over a predetermined phase width in units of predetermined length , wherein the apparatus comprises the following means.
That is, the apparatus has discriminating means for comparing a predetermined threshold value with a correlation value calculated in an interval shorter than a predetermined length at the time of calculation of a correlation value, for executing the calculation of the correlation value in a whole interval of the predetermined length only in the case where the calculated correlation value exceeds the threshold value, and for stopping the calculation of the correlation value in the case where the correlation value calculated in the interval shorter than the predetermined length does not exceed the threshold value.
As for the phase of a small correlation value, consequently, the correlation calculation can be stopped in a time shorter than the ordinary time without calculating the threshold value over the whole interval of the predetermined length, so that a reduction of the time which is required for calculation with regard to the whole predetermined phase width can be realized.